Screw-in electrode probes are known from public prior uses or may also be inferred from US 2002/0188340 A1 or US 2003/0144722 A1. They have an oblong electrode body made of an electrically insulating material, a rotatable electric supply line running in the electrode body, and an electrode head situated at the distal end of the electrode body. The latter is provided with the housing from which a corkscrew-like screw-in electrode may be screwed into cardiac tissue through rotation around its coil axis with corresponding axial displacement. For this purpose, the screw-in electrode is seated coaxially on a shaft which is in turn connected in its axial extension to the supply line for the electrode—typically a coiled wire—running in the electrode body. A torque may be transmitted to the shaft via the supply line, through a corresponding pitch provider, the shaft is then rotated around its longitudinal axis and axially displaced in order to screw the screw-in electrode actively into the cardiac tissue or retrieve it therefrom again.
Depending on the application, it may be necessary, for reasons of signal or stimulation technology, to produce an electrical connection between the electrically conductive housing, which is made of metal, and the screw-in electrode itself, so that both elements are at the same electrical potential. This is typically implemented by a contact spring, for which different embodiments and installation possibilities are known through public prior use. It is to be ensured in principle in connection with the active fixing and the rotation of the screw-in electrode, with its shaft, and the simultaneous axial displacement connected thereto that the contact spring is also capable of compensating for radial displacements in addition to the axial movement of shaft and screw-in electrode. For this purpose, welding both ends of a type of clock spring, as is used for the balance wheel of a clock, for example, to the shaft and/or the housing is known. Because of the filigree configuration of this spring, this assembly step is relatively complex and sensitive in regard to the reliability of the contact connections. Furthermore, this type of spring requires a relatively large installation space, since the displacement path, which must cover the fixed connection points of the clock spring on the housing and shaft in relation to one another, is comparatively large, specifically two rotations at an axial displacement of 2 mm, for example. This path must be provided by the length of the spring leg without building up a noticeable counter tension during the rotation and axial displacement of the screw-in electrode.
A further known possibility is to use a coiled compression spring between the housing and a corresponding stop on the shaft or screw-in electrode. A system of this type requires a relatively large installation space in the axial direction, however. Furthermore, the spring force of the coiled compression spring counteracts the insertion of the screw-in electrode when it is actively withdrawn into the housing, which obstructs the electrode actuation.
Finally, using a type of torsion spring between the electrode shaft and the housing for the connection has already been attempted. Because of the small installation space available and the corresponding filigree dimensioning, this contact spring only has a very slight contact force, however, so that contact stability may not be ensured, particularly with the radial oscillations of the coil typical for screw-in electrodes.